1. Field of the Invention
The present invention relates to gasket and sealing materials, and especially gasket materials pre-formed into various shapes for joining sealing faces (e.g., flat faced, standing-gap, or O-ring flanges) together to restrict fluid flow through a joint, such as those employed in distillation columns, glass lined vessels, and transformer radiator flanges.
2. Description of Related Art
Gaskets are used in the joining of piping, vessels, and such enclosures where fluid seals are required between opposing sealing faces (e.g., flange faces). There are three common different mechanical situations created when sealing two flanges together, and each requires special gasket design and intrinsic material properties. The first of these is a flat faced flange where opposing flange faces do not contact one another and are separated by the gasket material. Typically gasket materials for these flanges are not constrained from expanding or "creeping" in a direction parallel to the flange faces. The second type of flange configuration is considered a standing gap flange. In a standing gap flange there is partial flange face to flange face contact but the flanges do not completely contain the gasket material on all surfaces (thus allowing creep to occur only in the unconstrained direction). Lastly, an O-ring flange has complete flange face to flange face contact. The gasket sits in a groove on at least one of the flanges and is constrained from movement on all surfaces.
Each flange configuration requires specific gasket properties designed to assure fluid tight seals. With two flanges contacting each other, such as the O-ring or the standing gap flanges, there exists flange-to-flange contact that allows the flange to carry the majority of the bolt load and external loads that may be exerted on this joint. In these cases, the gasket material must be resilient, maintaining shape in the absence of constant load on the gasket. In a flat faced flange, the gasket bears the entire bolt and external load and must be resistant to compressive creep. In all flange sealing situations, the integrity of the seal is dependent on the gasket design and properties.
An example of flat faced flange is the seal between segments in a distillation column used in the petroleum industry. Here large segments of metal or glass cylinders are stacked vertically together with a gasket required to seal between each cylinder.
Standing gap flanges are encountered when sealing glass lined structures, such as glass lined metal dome lids to glass lined metal vessels. In this case, metal-to-metal contact on the exterior metal structure is required to support the mechanical loads of the flange bolts and compressive load of the dome lid and vessel. However, the gasket must be positioned carefully between the internal glass linings of the vessel and dome cover so as not to damage the glass, yet it must provide a tight seal between the glass faces and the metal flanges. The gasket material is not constrained on its interior surface and does not bear the full compressive load of the bolts and glass lined vessel and dome. The gasket, therefore, must resist creep on its unconstrained interior surface.
An example of an application of O-rings is in radiator piping of large electric transformers. A machined metal groove in one flange face provides a cavity for the gasket material to rest with about 20-40% of its height protruding above its groove. When the second flange face is brought in contact with the first, the gasket material is compressed to form a tight seal. In this manner, the gasket material is constrained on all surfaces.
It should be understood that opposing flange faces can be made of a wide variety of materials (e.g., metal, glass/ceramic, plastic, etc.), comprising mated sealing surfaces that are either homogeneous or heterogeneous combinations of materials. Additionally, a given flange face can be constructed of heterogeneous materials, such as the case where one flange face has both metal and glass sealing surface. This situation compounds the difficulty of producing a tight and durable seal between flanged joints.
Elastomer materials such as butyl rubber, neoprene, ethylene propylene rubber (EPDM), nitrile rubber, and cork/elastomer or asbestos gaskets are commonly used in flange sealing applications, but each of these has limitations or drawbacks. For instance, a sometimes difficult compromise must be struck between a material that provides a seal and a material that is durable and chemical/heat resistant for long-term use. Another common constraint is the inability of gaskets to compensate for misaligned, bent, uneven, corroded, or otherwise defective flange faces, or as mentioned, flanges made from heterogeneous materials.
Elastomeric materials are desired from the standpoint of their resiliency and compressive counterforce. However, elastomers' long-term performance limitations when in the presence of elevated temperatures, ultra-violet radiation, or aggressive chemicals make them less than ideal for demanding applications, such as transformer radiator flanges, and glass lined vessels and distillation columns. What is required is a sealant that tightly fills the flange joint and maintains a compressive counterforce, yet is unaffected by thermal or chemical exposure.
One material that has superior heat and chemical resistant properties is polytetrafluoroethylene (PTFE). As a gasket, PTFE has exhibited utility as a material for use in harsh chemical environments that normally degrade many conventional metals, elastomers, and polymeric materials. Conventional, full density PTFE has a usable temperature range from as high as 260.degree. C. to as low as near -273.degree. C.
However, conventional non-porous PTFE gasket materials which have been compression molded or extruded and then heated to a temperature above 345.degree. C. exhibit poor mechanical properties, such as low tensile strength and poor creep resistance. This limits or excludes the use of such materials in these applications requiring long term resistance to creep.
PTFE may be produced in an expanded, porous form as taught in U.S. Pat. No. 3,953,566 issued Apr. 27, 1976, to Gore. Expanded polytetrafluoroethylene (ePTFE) is of a higher strength than conventional PTFE, has the chemical inertness of conventional PTFE, and has an increased temperature range of up to about 315.degree. C. in service. An example of a porous expanded PTFE gasket material is available from W. L. Gore & Associates, Inc., of Elkton, Md., under the trademark GORE-TEX.RTM. Joint Sealant.
Although polytetrafluoroethylene (PTFE) can provide the necessary durability, its tendency to experience compressive creep and inability to conform to shaped surfaces renders this material problematic as well. Virtually all PTFE gaskets (virgin, filled, or expanded) exhibit varying degrees of compressive creep or flow. With metal-to-metal contact of the flanges in standing gap and O-ring joint flanges, there is no mechanism for compensating for even a slight amount of creep. If the gasket creeps and, as a result, becomes thinner, there is no longer a counterforce being exerted by the gasket against the flanges.
One suggestion for achieving the chemical resistance of PTFE but limiting the amount of creep of the material is to coat a generally creep-stable material such as synthetic rubber with a coating of PTFE to provide chemical resistance. One example of such a structure is presented in U.S. Pat. No. 4,898,638 issued Feb. 6, 1990, to Lugez. In this patent it is taught that through a disclosed process one or more films of only partially porous PTFE can be adhered to a rubber sheet to provide a gasket material with chemical resistance. While this approach may addresses some of the problems with existing elastomer sealing materials, the PTFE film can crack under the stresses of compression, leading to exposure and potential failure of the core elastomer. It is believed that longer life and better thermal and chemical resistance are possible if a PTFE material is employed throughout the cross-section of the gasket.
As is disclosed in co-pending U.S. patent application Ser. No. 050,903, filed Apr. 20, 1993, it has been determined that a PTFE sealing material can be produced with limited long-term creep by wrapping a core of elongated or expanded PTFE with a high strength film of expanded PTFE. The high strength film is resistant to deformation and stretching and serves to contain the PTFE core in place within a compressed gasket. This material has proven to be quite effective in sealing plate and frame heat exchangers and the like, thereby providing thermal and chemical protection, long-life and durability, and ease in replacement. However, it is believed that with modifications such material may be useful in wide variety of other sealing environments.
Accordingly, it is a primary purpose of the present invention to provide a gasket material for sealing flanged apparatus which provides an effective long-term seal under pressure or vacuum, while being durable, conformable, chemical and thermal resistant, non-contaminating, and easy to install.
It is still another purpose of the present invention to provide a gasket material for sealing a wide range of flanges that can utilize the benefits of PTFE or expanded PTFE material, while avoiding the problem of compressive creep and gasket failure.
It is a further purpose of the present invention to provide a method for making and optimally using a gasket material with the above properties.
These and other purposes of the present invention will become evident from review of the following specification.